A surge protector or arrester is commonly connected across a comparatively expensive piece of electrical equipment to shunt over-current surges. Such over-current surges occur, for example, when lightning strikes. When this happens, the surge arrester shunts the surge to ground, thereby protecting the piece of electrical equipment and the circuit from damage or destruction.
Present day surge arresters commonly include an elongated, hollow cylindrical housing made of porcelain or the like, and a plurality of non-linear resistive blocks within the housing. Some of these structures also include spark gaps, the blocks and gaps being electrically interconnected to handle voltage and current surge conditions arising on a power line. The blocks commonly contain silicone carbide (SIC) or metal oxide varistors (MOV), and are usually in the shape of relatively short cylinders stacked within the arrester housing. The number of blocks employed is a function of the material (SIC or MOV) and the voltage and current ratings of the assembly.
For a surge arrester to function properly, intimate contact must be maintained between the MOV or SIC blocks. This necessitates placing an axial load on the blocks within the housing. Prior art arresters utilize bulky contact springs within the housing to provide this axial load. Typically, these springs can provide only relatively small loads, for example, about sixty pounds. As a result, prior art surge arresters experience one or more problems such as poor heat transfer between the MOV or SIC blocks and arrester terminals; non-uniform current distribution; and high contact resistances at joints. Furthermore, units having low contact force sputter and the ionized metal which is produced can cause axial flashover at high currents.
An additional problem with surge arresters of the prior art is that they, on rare occasions, fail in a dangerous fashion. When these arresters fail and experience high fault currents producing high internal gas pressures, the bursting unit may throw parts and cause property damage.
In addition, some of the prior art devices are difficult to assemble, have poor dielectric design, are susceptible to water invasion, and require totally different devices to provide varied voltage ratings.
Examples of prior art surge arresters are disclosed in the following U.S. Pat. Nos. 2,587,587 to Bellezza et al; 2,947,903 to Westrom; 2,997,529 to Fink; 3,018,406 to Innis; 3,261,910 to Jacquier; 3,412,273 to Kennon et al; 3,524,107 to Reitz; 3,566,183 to Olsen; 3,567,541 to Kaczerginski; 3,586,934 to Nakata; 3,706,009 to Reitz; 3,725,745 to Zisa; 3,850,722 to Kreft; 3,973,172 to Yost; 3,987,343 to Cunningham et al; 4,029,380 to Yonkers; 4,092,694 to Stetson; 4,100,588 to Kresge; 4,107,567 to Cunningham et al; 4,161,012 to Cunningham; 4,218,721 to Stetson; 4,404,614 to Koch et al; 4,467,387 to Bergh et al; 4,491,687 to Kaczerginski et al; and U.S. Defensive Publication No. T102,103, as well as U.K. Pat. No. 730,710; 1,109,151; and 1,505,875.
In the surge arresters of commonly assigned U.S. Pat. No. 4,656,555 to Raudabaugh, copending U.S. patent application Ser. No. 033,765 of Donald E. Raudabaugh entitled Polymer Housed Electrical Assemblies Using Modular Construction and filed Apr. 3, 1987, and copending U.S. patent application Ser. No. 176,319 entitled Modular Electrical Assemblies with Plastic Film Barriers of Donald E. Raudabaugh and filed Mar. 31, 1988, the subject matters of which are hereby incorporated by reference, epoxy soaked glass fibers surrounding and axially compressing the varistor blocks and end terminals are located in end terminal grooves closed in an axially outward direction by end terminal portions. When the epoxy cures, the wrap shrinks and does not completely fill the end terminal grooves, adversely affecting the ability to withstand cantilever loading.